Mitogen regulated protein (MRP) was first discovered as a heterogeneously glycosylated protein produced by 3T3 cells in response to stimulation by growth factors such as epidermal growth factor and fibroblast growth factor. It is produced in vivo by the trophoblastic giant cells in the murine placenta. MRP is a member of the prolactin/growth hormone family. The period of production of MRP in gestation coincides with a period of rapid fetal growth and is the peak period of organogenesis. We have discovered a high affinity, specific receptor for MRP. The period of expression of the receptor in gestation is brief and coincides with the period of MRP expression. We believe MRP plays a role in regulating maternal and perhaps fetal growth at midgestation. We propose to investigate the physiological role of MRP by 1) cloning its receptor using expression in COS cells, and 2) determining the cellular response to MRP in cell cultures and looking for evidence of a function regulated by MRP in vivo. We expect these studies to provide us with information about the structure and function of the MRP receptor and to reveal a physiological role for MRP in fetal and maternal growth and development.